On Interstate highways in
the United States, speed limits range from urban limits as low as 40 mph
(65 km/h) to rural limits as high as 75 mph (120 km/h). Before the 1973
energy crisis, some states posted no speed limit on Interstate highways.
At one time Kansas had an 80 mph (130 km/h) speed limit on its turnpike
system. In 1974 Congress imposed a nationwide 55 mph (90 km/h) speed
limit by threatening to withhold highway funds from states that did not
adopt this limit. It was estimated a speed of 55 mph used 17% less fuel
per mile than a speed of 75 mph (120 km/h). It was also believed, based
on a noticeable drop the first year the limit was imposed, that it cut
down on highway deaths, but later studies were more mixed on this point.
This limit was unpopular, especially in Western states. In 1987 states
were permitted to raise speed limits to 65 mph (105 km/h) on rural
All federal speed limit controls were lifted on November 28, 1995,
leaving the task of setting maximum speeds to the states. Immediately,
Montana reverted to non-numerical speed limits on most rural highways.
Many other states reverted to older policies allowing speed limits
higher than 65 mph (105 km/h). However, no Interstate highway, freeway,
or expressway is currently signed for over 75 mph (120 km/h), and within
major city limits, few freeways have speed limits over 65 mph (105
In California many speed limit signs are identified as "Maximum Speed",
usually when the limit is 55 mph (90 km/h) or more.
The Kansas Turnpike used to have an 80 mph speed limit. No Kansas road
currently has a speed limit higher than 70 mph.
It is a myth that Montana had no speed limit. In fact, for four years,
it had a "reasonable and prudent" speed limit during the daytime on most
rural roads. At night, these roads had a night speed limit, usually 65
MPH or 55 MPH, depending on road type. In some cases, the police
enforced an approximately 90 mph (140 km/h) speed limit as "reasonable
In a challenge to a speeding ticket, the "reasonable and prudent" laws
was found to be too vague to be constitutional. Thus, for most of the
first half of 1999, there was no speed limit whatsoever on most rural
In June 1999, a new Montana speed limit law went into effect. The law's
practical effect was to require posted limits on all roads and disallow
any speed limit higher than 75 mph (120 km/h).
Typical Texas rural speed limit sign. Note the black back grounded 65
mph night speed limit sign, common on Texas roads. (Few other states
have widespread night speed limits.) This sign is on southbound U.S.
69/96/287 just north of Beaumont.
Texas is the only state whose speed limit laws and rules generally do
not prescribe a specific limit for each type of roadway. Any rural
road—two lane, four lane, Interstate, or otherwise—that is numbered by
the state or federal government has a 70 mph (110 km/h) statutory limit.
The law generally allows changing the 70 mph limit only if a study
recommends a different limit.
75 mph and 80 mph limits
In 2001, the Texas Legislature allowed the Texas Department of
Transportation to post 75 mph (120 km/h) speed limits in counties with
fewer than 10 people per square mile. This has the practical effect of
only allowing 75 mph speed limits in the most sparsely populated
counties, all of which are generally well west of a line stretching from
San Antonio to Odessa. In 2005, the Texas Legislature revised this law,
allowing 80 mph (130 km/h) limits on I-10 and I-20 in certain rural
counties in west Texas. This law also revised the eligibility for 75 mph
speed limits: now eligible counties can have up to 15 persons per square
Texas law does not disallow 75 mph speed limits on two-lane roads.
Several west Texas 2 lane roads carry 75 mph limits, including portions
of US 90. No other state has a limit higher than 70 mph on any 2 lane
Environmental speed limits
Texas is the first state to lower speed limits for purportedly
environmental reasons since the 1995 repeal of federal speed limit
controls. In roughly a 50-mile (80 km) radius of the Houston-Galveston
and Dallas-Ft. Worth regions, the Texas Commission on Environmental
Quality co-opted the Texas Department of Transportation to reduce the
speed limit on all roads with 70 mph (110 km/h) or 65 mph (100 km/h)
speed limits by 5 mph. This was instituted as part of a plan to reduce
smog-forming emissions in areas out of compliance with the federal Clean
Initial studies suggested the lower speed limits could bring the areas
roughly 1.5% closer to compliance. However, follow-up studies suggest
that the actual benefit is only a fraction of this original estimate.
First, the emissions modeling software used for intitial estimations,
MOBILE 5a, overestimated the emissions contribution of speed limit
reductions. Rerunning the models with the next generation software,
MOBILE 6, produced dramatically lower emissions reductions. Second,
speed checks in the Dallas area performed 1 year after implementation of
speed limit reductions show that actual speed reductions are only about
1.6 MPH, a fraction of the anticipated 5.5 MPH speed reduction. With
both of these facts combined, it is possible that the speed limit
reductions only provide a thousandth of the total emissions reductions
necessary for Clean Air Act compliance.
The Houston-Galveston area briefly had all roadways capped at 55 mph (90
km/h) in mid-2002. Facing immense opposition, poor compliance, and the
realization that lowered speed limits produced only a fraction of the
originally estimated emissions reductions, the TCEQ relented and
reverted to the 5 mph reduction scheme.
In 2003, the Texas Legislature prospectively banned environmental speed
limits effective September 1, 2003. The wording of the bill allows
environmental speed limits already in place to remain indefinitely; no
new miles of roadway may be subjected to environmental speed limits,
This law has allowed interesting inconsistencies. Generally, all primary
arterial roadways within the inner loops of Texas cities have speed
limits of 60 mph (95 km/h) or lower, so they were not subjected to
environmental speed limits. Arterial roads between the inner loop and
the outer loop generally have 65 mph (100 km/h) limits, and arterial
roads outside the outer loop generally have 70 mph (110 km/h) limits.
(Note that this "standard" is only an observed pattern. It is not
prescribed by law.) In at least one case—TX 121 between I-35W and I-820
in Ft. Worth—the speed limit rises from 60 mph to 65 mph as one crosses
IH-820 approaching downtown, contravening the standard.
Night speed limits
Texas is the only state with a broadly applicable night speed limit.
Texas statutorily prescribes a blanket 65 mph (100 km/h) night speed
limit on roads with a speed limit of at least 70 mph (110 km/h). While
the Texas Department of Transportation has the power to raise or lower
this night speed limit, it in fact rarely does, so nearly every 70 mph
or higher speed limit sign has an accompanying 65 mph night speed limit
North Dakota, Montana, and Oklahoma also have night speed limits, but
they are only generally applicable to rural, non-Interstate-class roads.
In addition to the legally defined maximum speed, minimum speeds may be
posted. There is little evidence to suggest they are enforced. In
California the minimum speed by regulation on freeways (in free flowing
traffic) is 45 mph, although this is generally not posted.
On US roads, the speed limits are usually as follows:
* 15–25 mph (25–40 km/h) in school zones
* 25–30 mph (40–50 km/h) on residential streets in cities and towns
* 35–45 mph (55–70 km/h) on major arterial roads in urban and suburban
* 45–70 mph (70–110 km/h) on highways outside cities and towns and urban
* 55–70 mph (90–110 km/h) on non-Interstate freeways and rural
* 65–75 mph (105–120 km/h) on rural Interstate freeways
Generally, western states have higher limits than eastern states.
For a current listing of all U.S. State Highway Speed Limits click on
Speed Limits by State, Insurance Institute for Highway Safety